**What is Erlang: Unit of Traffic**

An Erlang is a unit of telecommunications traffic measurement. Strictly speaking, an Erlang represents the continuous use of one voice path. In practice, it is used to describe the total traffic volume of one hour.

For example, if a group of user made 30 calls in one hour, and each call had an average call duration of 5 minutes, then the number of Erlangs this represents is worked out as follows:

Minutes of traffic in the hour = number of calls x duration

Minutes of traffic in the hour = 30 x 5

Minutes of traffic in the hour = 150

Hours of traffic in the hour = 150 / 60

Hours of traffic in the hour = 2.5

**Traffic figure = 2.5 Erlangs**

Erlang traffic measurements are made in order to help telecommunications network designers understand traffic patterns within their voice networks. This is essential if they are to successfully design their network topology and establish the necessary trunk group sizes.

Erlang traffic measurements or estimates can be used to work out how many lines are required between a telephone system and a central office (PSTN exchange lines), or between multiple network locations.

**Erlang Traffic Theory:**

When half rate is used it will theoretically double the number of available traffic channels. In

practice, however, live networks will most likely consist of a mixture between half rate mobiles

and full rate mobiles.

Half rate will affect the SDCCH dimensioning since more Signalling will be required when the number of TCHs is increased. An important dimensioning factor is therefore the half rate penetration, i.e. the percentage of half rate mobiles in the network.

When half rate TCH capacity calculations are done it is assumed that the half rate mobiles are evenly spread among the cells, i.e. all cells have the same half rate penetration. The TCH capacity calculations made in this guideline are best illustrated with an example:

If for example a 2 TRX cell is used, it can accommodate 14 full rate TCHs, i.e. 14 subscribers if one SDCCH/8 is used for Signalling. A half rate penetration of 10 % would mean that 10 % of the 14 subscribers would be using a half rate connection, i.e. 1.4 subscribers (after been rounded up = 2 subscribers). This would result in 13 timeslots used for full rate and 1 timeslot used for half rate, resulting in 13 full rate TCHs and 2 half rate TCHs, i.e. 15 TCHs in total.

The TCH capacity is then calculated for 15 TCHs with an Erlang B table with appropriate

blocking figure.

*Knowing the SDCCH holding times, with a given number of performances during busy hour for every procedure, the generated SDCCH traffic per subscriber can be calculated as follows: For each type of procedure, multiply the number of performances per busy hour and subscriber by the holding time of the channel. By dividing the result by 3.6, the procedures contribution to the SDCCH load in mErlang/subscriber is achieved.*